Grammar Girl here.

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Today’s topic is illegal versus unlawful.  Here's a question from Jed in Washington, D.C.

From my seat on the bus, I could see a big sign listing things that were "unlawful" to do on the bus (such as eat, listen to loud music, etc.)  I was curious if this word carried less force than illegal, even though they both seem to mean the same thing according to a few dictionaries that I checked.

Thanks Jed! I have some language-related comments, but I'm bringing in Legal Lad to answer the meat of your question.

Legal Lad:

Great question, Jed. The short answer is that there is a slight semantic difference between the two words, but no difference with regard to criminal punishment. 

Grammar Girl:

The prefixes il- and un- both mean the same thing—they mean not. So do both of these words mean not lawful?

Legal Lad:

Black’s Law Dictionary defines unlawful as not authorized by law, illegal. Illegal is defined as forbidden by law, unlawful. Semantically, there is a slight difference.  It seems that something illegal is expressly proscribed by statute, and something unlawful is just not expressly authorized.

Jaywalking is a good example of an unlawful act. Traffic regulations do not typically say that you cannot walk diagonally through an intersection. So, it is not illegal. Rather, traffic regulations typically provide that you can cross within a crosswalk when the little walky-man appears. Crossing in any other way is unlawful because it is not expressly permitted.

Selling cocaine is a good example of an illegal act. A federal law specifically provides that you may not do so.

With regard to Jed’s question, it would depend on point of view. On one hand, the Washington Metropolitan Area Transit Authority, aka Metro, issued a rule that prohibits eating or drinking while riding on a public bus. So, the act is expressly proscribed, and thus illegal.

On the other hand, Metro is not a legislative body and does not pass laws in the traditional sense. Rather, it was a body created by an Interstate Compact in 1967. Part of the compact was that Metro could create rules to ensure safe and comfortable transportation for the public, and Metro used that authority to make a rule against eating or drinking. But, the compact, the actual law, does not say anything about food; it only says that the agency could create rules for safe travel. Thus, eating and drinking is simply not permitted, and thus unlawful.
 
Practically, there is no difference for punishment purposes. Both illegal and unlawful acts can get you into trouble.

Grammar Girl:

Interesting! So Jed had better not eat and rock out on the bus.

I found a couple of interesting things while I was reading about prefixes. First, un- (as in unlawful) is an English prefix, and in- (as in injustice) is the corresponding Latin prefix.

And then second, il- (as in illegal, illicit, and illegitimate) is considered to be a form of the prefix in- (as in injustice and indivisible).

It works a little bit like how you choose to use the words a or an depending on whether the next word starts with a consonant or vowel sound. In this case, the prefix in- gets changed to il- when the word starts with the letter l, and it also gets changed to im- when the word starts with a p or b, as in impossible and imbalance.

There's bonus information on the Grammar Girl website about the prefix in-.

Web Bonus

The prefix in- has two meanings.

First, it can roughly mean the equivalent of the English word in, as in inclusive and inland. This version comes from Old English and Latin.

Second, it can roughly mean not, as in injustice and indivisible. This version comes from Latin. 

 That's all.

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If you visit QuickAndDirtyTips.com you can find all my contact information and read the entire transcript of this podcast. While you're there, be sure to check out the other great podcasts such as Modern Manners. This week he's answering a tough question about how to deal with uninvited guests.   

Thanks for listening.

References

1. “In- as a Prefix,” The Maven's Word of the Day, New York: Random House, June 4, 1997 www.randomhouse.com/wotd/index.pperl?date=19970604 (accessed July 25, 2007).
2. Wilson, K.G. “il-,”  The Columbia Guide to Standard American English. New York: Columbia University Press, 1993, www.bartleby.com/68/6/3106.html (accessed July 25, 2007).
3. in-. Dictionary.com. Dictionary.com Unabridged (v 1.1). New York: Random House, Inc. dictionary.reference.com/browse/in- (accessed: July 26, 2007).
4. in-. The American Heritage® Book of English Usage. Boston: Houghton Mifflin Company, 1996
   http://www.bartleby.com/64/pages/page247.html (accessed July 25, 2007).
5. Xavier, J. “How Negative Prefixed Are Determined in English,” The Linguistic Zone, June 19, 2007, http://urltea.com/1ee9 (accessed July 26, 2007).